Method for producing resin-molded semiconductor device having heat radiating plate embedded in the resin

ABSTRACT

A lead frame has a plurality of metallic heat radiating plates, leads connected to each metallic heat radiating plate, other leads separated from the metallic heat radiating plates and a common insulator web attached to the metallic heat radiating plates. When the lead frame is placed in the cavity of a mold, the metallic heat radiating plates are correctly positioned in the mold cavity by means of the insulating web and the connected leads. By effecting a resin molding under this condition, the metallic heat radiating plates are molded in resin such that a layer of resin with a uniform thickness cover the back surface of the respective metallic heat radiating plates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a resin-molded semiconductor deviceand, more particularly, to a construction of a resin-moldedsemiconductor device attachable directly to an external heat radiatingfin.

2. Description of the Prior Art

In a typical semiconductor device in the prior art,a semiconductorpellet having a transistor formed thereon is attached to a predeterminedportion of a metallic heat radiating plate. A collector lead is securedto the metallic heat radiating plate by, for example, welding, while theemitter and base leads are arranged such that they are positioned nearthe metallic heat radiating plate. The emitter and the base electrodesof the semiconductor pellet are connected to the emitter and base leadsby bonding fine metallic wires. Then, the vicinity of the heat radiatingplate including tips of the emitter and base leads are covered by amolded resin. In the production of this type of resin-moldedsemiconductor device, the back surface of the metallic heat radiatingplate opposite to the surface on which the semiconductor pellet isattached is held in contact with the bottom of the mold during the resinmolding process, so that the back surface of the metallic heat radiatingplate is exposed through the molded resin in the final product.

Therefore, when this resin-molded semiconductor device is attached to anexternal heat radiating fin, it is necessary to interpose a thininsulating film between the exposed back surface of the metallic heatradiating plate and the external heat radiating fin. In the recentyears, to obviate this trouble, a resin-molded semiconductor device hasbeen devised such that the whole surface of the heat radiating plate iscovered with the molded resin to permit a direct attaching of thesemiconductor device to the external heat radiating fin.

The production of such a resin-molded semiconductor device, however,encounters a difficulty in keeping the heat radiating plate afloat inthe mold cavity to cover the back surface of the heat radiating platewith the resin. To this end, the end of the heat radiating plateopposite to the end to which the collector lead is connected is thinnedas much as possible and made to project over a small width, and theresin molding is conducted while cantilevering the heat radiating plateat the projected end by means of the mold. Thus, the heat radiatingplate partially projects out of the molded resin in the final product.Therefore, when this resin-molded semiconductor device is mounted on theexternal heat radiating fin, an atmospheric discharge tends to takeplace between the heat radiating fin and the projected portion of themetallic heat radiating plate.

According to another known method of producing this type of resin-moldedsemiconductor device, the molding is conducted while supporting themetallic heat radiating plate afloat by means of pins which areprojected into the mold cavity. In this case, holes are left in themolded resin after the withdrawal of the supporting pins, and the heatradiating plate is exposed through the holes. Consequently, atmosphericdischarge tends to be caused between the exposed portions of themetallic heat radiating plate and the external heat radiating fin.

In order to prevent the atmospheric discharge, it has been proposed toapply an insulating paint to the projected or exposed portions of theheat radiating plate. The insulating paint, however, cannot providesufficient dielectric strength and, hence, cannot prevent theatmospheric discharge perfectly.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the present invention is to provide aresin-molded semiconductor device in which the metallic heat radiatingplate supporting the semiconductor pellet is not exposed through themolded resin.

Another object of the present invention is to provide a method forproducing a resin-molded semiconductor device in which the metallic heatradiating plate is not exposed through the molded resin, wherein themetallic heat radiating plate is correctly positioned and oriented inthe resin during the molding process.

To these ends, according to an aspect of the invention, there isprovided a resin-molded semiconductor device comprising: a semiconductorpellet; a heat radiating plate carrying the semiconductor pellet; aninsulator web attached to the metallic heat radiating plate and extendedover both sides of the metallic heat radiating plate; means for leadingthe electrodes of the semiconductor pellet to the outside; and a resinmember which covers the semiconductor pellet, the metallic heatradiating plate, a part of the means for leading the electrodes to theoutside and a portion of the insulator web except a part of the surfaceof the latter, the part of the surface of the insulator web beingexposed in the surface of the resin member.

According to another aspect of the invention, there is provided a methodfor producing a semiconductor device comprising the steps of: attachingan insulator web to a metallic heat radiating plate such that theinsulator web extends over both sides of the metallic heat radiatingplate; attaching a semiconductor pellet to the metallic heat radiatingplate; providing means for leading the electrodes of the semiconductorpellet to the outside; placing the metallic heat radiating plate, thesemiconductor pellet, a part of the insulator web and a part of leadingmeans in the cavity of a mold so as to clamp the other portion of theinsulator web and the other portion of the means for leading theelectrodes by means of the mold thereby to locate the metallic heatradiating plate in the mold; charging a resin into the cavity of themold; solidifying the charged resin and taking out the solidified bodyfrom the mold; and removing by cutting the insulator web projecting fromthe surface of the solidified resin.

The semiconductor device of the invention exposes no conductive partexcept the means for leading the electrodes to the outside through themolded resin. Therefore, the undesirable atmospheric discharge is notgenerated between the metallic heat radiating plate and the externalheat radiating fin to which the semiconductor device is attached. It is,therefore, possible to obtain a semiconductor device having a highdielectric strength and, hence, operable at a high voltage.

Furthermore, in the resin molding process, the metallic heat radiatingplate is located correctly within the mold cavity by means of a web-likeinsulator so that the metallic heat radiating plate occupies thepredetermined position in the resin. It is, therefore, possible touniformalize the thickness of the molded resin under the metallic heatradiating plate and, accordingly, undesirable reduction of dielectricstrength attributable to a lack of uniformity of resin thickness isavoided advantageously.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a sectional view of a resin-molded semiconductor device in theprior art;

FIG. 2 is a sectional view of another resin-molded semiconductor devicein the prior art;

FIG. 3 is a sectional view of still another resin-molded semiconductordevice in the prior art;

FIG. 4a is a partial plan view of a lead frame used in a preferredembodiment of the present invention;

FIG. 4b is a sectional view taken along the line B-B' of FIG. 4a;

FIG. 5 is a plan view showing the relationship between the lead frameand a mold in the resin molding process in accordance with a preferredembodiment of the present invention;

FIG. 6 is a sectional view taken along the line Z-Z' of FIG. 5;

FIG. 7a is a sectional view of a preferred embodiment of theresin-molded semiconductor device in accordance with the presentinvention; and

FIG. 7b is a sectional view taken along the line Y-Y' of FIG. 7a.

DETAILED DESCRIPTION

FIG. 1 shows an example of a resin-molded semiconductor device in theprior art. This semiconductor device is provided by a process having thesteps of preparing a metallic heat radiating plate having a lead 6attached to one end thereof and base and emitter leads (not shown),securing a semiconductor pellet 2 to said metallic heat radiating plate1, connecting the base and emitter leads to the electrodes of thesemiconductor pellet 2 by means of fine metal wires, placing themetallic heat radiating plate 1 in the mold cavity, pouring a resin 5into the mold cavity and solidifying the resin. The mold has aprojection projecting into the cavity by means of which a threaded bore8 is formed in the resin body 5.

In the resin molding process, the metallic heat radiating plate 1 isheld in contact with the bottom of the mold cavity, so that the metallicheat radiating plate is exposed at the bottom of the productsemiconductor device. Therefore, for securing the semiconductor deviceto the external heat radiating fin, it is necessary to interpose a thininsulating plate such as of mica between the bottom of the semiconductordevice and the external heat radiating fin.

In order to eliminate this thin insulating film, proposed is aresin-molded semiconductor device in which the bottom surface of thesemiconductor device is entirely covered by the molded resin. However,since the metallic heat radiating plate has considerably large weight,the heat radiating plate is declined within the mold cavity if it ismerely floated in the resin within the mold cavity. Due to the declineof the metallic heat radiating plate 1, the thickness of the resin layerunder the metallic heat radiating plate 1 is made non-uniform as shownin FIG. 2. The dielectric strength between the metallic heat radiatingplate 1 and the external heat radiating fin is determined by thesmallest thickness of the resin under the metallic heat radiatingplate 1. Consequently, the dielectric strength of the semiconductordevice is decreased disadvantageously. This problem would be overcome byincreasing the resin thickness under the metallic heat radiating plate.This, however, will be accompanied by an increase of the size of thesemiconductor device as a whole.

FIGS. 2 and 3 show other examples of conventional resin-moldedsemiconductor devices improved to realize a uniform thickness of theresin 15 and 25 under the metallic heat radiating plate 11 and 21. Inthe case of the semiconductor device shown in FIG. 2, a lead 16 isattached to one end of the metallic heat radiating plate 11 to which asemiconductor pellet 12 is secured, while the other end of the heatradiating plate 11 is provided with a thin metallic projection 13.During the molding process, the projection 13 is held together with thelead 16 in the mold to stably hold the metallic heat radiating plate 11within the cavity, so that the molded resin 15 under the metallic heatradiating plate 11 in the final product can have a uniform thickness t.In this case, however, the metallic projection 13 is inevitably left toproject from the resin 15 in the final product. A reference numeral 17denotes fine metallic wires through which the electrodes on thesemiconductor pellet 12 are connected to leads which are not shown,while a numeral 18 designates a threaded bore for receiving a mountingscrew.

On the other hand, the conventional resin-molded semiconductor deviceshown in FIG. 3 has a metallic heat radiating plate 21 which has noprojection. A lead 26 is attached to one end of the metallic heatradiating plate 21. In the production of this semiconductor device, thesemiconductor pellet 22 is attached to the metallic heat radiating plate21, and the electrodes on the semiconductor pellet 22 are connected byfine metallic wires to the base and emitter leads which are not shown.The assembly is then placed in the mold cavity. The mold is providedwith small projections for supporting the metallic heat radiating plate21, besides large projections for forming a threaded hole in theproduct. The heat radiating plate 21 is held by the small projectionstogether with the leads, so that it can be correctly positioned withinthe mold cavity. Then, the resin is poured into the cavity andsolidified to form a resin member 25. In this case, however, holes 29and 29' are formed in the resin member 25 of the final product after thewithdrawal of the small projections of the mold for supporting the heatradiating plate 21, so that the metallic heat radiating plate 21 ispartly exposed through these holes 29 and 29'.

Thus, according to the semiconductor devices in the prior art, eitherthe metallic projection 13 projecting from the metallic heat radiatingplate 11 or the partial exposure of the metallic heat radiating plate 21through the small holes is inevitable, in order to attain a uniformthickness of the resin member under the metallic heat radiating plates11 and 21. When the conventional resin-molded semiconductor device shownin FIG. 2 or 3 is secured to the external heat radiating fin, anatmospheric discharge tends to occur between the projection 13 or theexposed portion of the metallic heat radiating plate 21 and the externalheat radiating fin, resulting in a reduction or lack of stability of thedielectric strength of the final product. In order to avert from thisproblem, it has been proposed also to apply an insulating paint on theprojection 13 or the exposed portions of the metallic heat radiatingplate 21. The prevention of the atmospheric discharge by the insulatingpaint, however, is not so reliable and cannot be used practically.

These problems in the prior art, however, can be overcome by the presentinvention as will be understood from the following description of thepreferred embodiments. FIGS. 4a and 4b show a lead frame used in apreferred embodiment of the present invention. The lead frame is formedby punching a metallic plate in the form of an integral member having aplurality of units 30₁, 30₂, . . . connected in series. Each unit has alead 36 for leading out the collector and leads 43 and 44 for leadingout the emitter and the base. The leads 36, 43 and 44 are connectedthrough transverse bars 41 and 42. The end of the lead 36 is secured by,for example, welding to a metallic heat radiating plate 31 having athreaded hole 38' formed therein. Each metallic heat radiating plate 31is made of a metallic plate such as of copper having a thickness of, forexample, 1.3 to 1.4 mm, and is provided at its end with a thin smallprojection 33 formed by press. The projection 33 projects from themetallic heat radiating plate 31 by a length of, for example, 1 mm. Aninsulator web 40 made from a glass epoxy resin and having a thickness of0.5 mm and a width of 0.6 mm is attached to all the projections 33 inthe units 30₁, 30₂, . . . by bonding or caulking. A semiconductor pellet32 of, for example, a transistor is attached to a predetermined portionof the heat radiating plate 31, and the electrodes on the semiconductorpellet 32 are connected to the leads 43 and 44 by bonding fine metallicwires 37.

The lead frame thus formed is placed in a mold 50, as shown in FIGS. 5and 6. The mold 50 is composed of an upper mold half part 50-1 and alower mold half part 50-2 which are adapted to form, when broughttogether, a resin runner 52 and cavities 51. The arrangement is suchthat the portion of the lead frame around the metallic heat radiatingplate 31 is placed in the cavity 51. According to this arrangement, theleads 36, 43 and 44 and the insulator web 40 are clamped by the moldhalf parts, so that the metallic heat radiating plate 31 is stably heldat a correct position in the cavity 51 by means of the lead 36 and theinsulator web 40. A pin 53 is projected from the upper mold half part50-1 downward into the cavity 51 to reach the lower mold half part 50-2through the threaded hole 38' formed in the metallic heat radiatingplate 36.

After placing this lead frame in the mold 50, epoxy resin is poured intothe cavities 51 through the resin runner 52, and is solidified in duecourse. After the solidification of the resin, the molded bodies aretaken out of the mold 50 and the transverse bars 41 and 42 are removedby cutting. Then, the insulator web 40 is cut at the side surface of theresin 35.

FIGS. 7a and 7b show sections of the thus severed semiconductor device.The resin member 35 is sized to be, for example, 10 mm in width, 17 mmin length and 4.5 mm in thickness, while the length of the leads 36, 43and 44 projecting from the resin 35 is 14 mm. The threaded hole 38 has adiameter of 3.2 mm at its upper end opening but the lower end opening ofthe same has a diameter smaller than the upper end opening.

In the thus produced semiconductor device, the resin member 35 under themetallic heat radiating plate 31 can have a uniform thickness of 0.35 to0.65 mm, preferably 0.4 mm to 0.5 mm, because the resin molding isconducted while correctly positioning the metallic heat radiating plate31 in the mold cavity 51. In the final product, only the cut surface ofthe insulator web 40 besides the leads 36, 43 and 44 is exposed on thesurface of the resin member 35, so that the undesirable atmosphericdischarge can be avoided when the semiconductor device is attached tothe external heat radiating fin. The maximum voltage applicable to themetallic heat radiating plate 31 is determined by the dielectricstrength of the resin member 35 under the metallic heat radiating plate31. In addition, the resin-molded semiconductor device of the presentinvention can be produced merely by adding a simple step to theconventional process for producing the resin-mold semiconductor deviceshown in FIG. 1. This additional step can be made without requiring anystrict control.

Although the present invention has been described through a preferredembodiment applied to a semiconductor pellet having a transistor, thisis not exclusive and the invention can equaly be applied to otherdevices such as a power integrated circuit incorporating a metallic heatradiating plate. The use of the glass epoxy resin as the material of theinsulator web 40 is not exclusive. Namely, the insulator web may beformed from other insulator material than the described glass epoxyresin, provided that the material has a softening point higher than thetemperature at which the resin member 35 is molded. The insulator web 40may be directly attached to other portions of the metallic heatradiating plate than the projection 33. Silicone resins and otherinsulative resins can be used in place of the epoxy resin used as thematerial of the resin member 35.

What is claimed is:
 1. A method for producing a semiconductor devicecomprising the steps of:preparing a lead frame in which united are aplurality of units each having a plurality of leads for externalconnection and a metallic heat radiating plate secured to one of saidleads; securing a common insulator web to one end of said metallic heatradiating plates of said lead frame; securing a semiconductor pellet toeach of said metallic heat radiating plate of said lead frame;electrically connecting the electrodes of said semiconductor pellet tosaid leads of said lead frame; placing said lead frame having saidsemiconductor pellets secured thereto and having said leads connected tosaid electrodes of said semiconductor pellets in a cavity of a mold;positioning said metallic heat radiating plates in said mold cavity byclamping said leads and said insulator web by said mold; charging aresin into said mold cavity; taking out said lead frame from said moldafter the solidification of said resin; and cutting said lead frame andsaid insulator web to severe said lead frame into individualsemiconductor devices.
 2. A method for producing a semiconductor deviceas claimed in claim 1, wherein said insulator web is cut along thesurface of the solidified resin so that the surface of said insulatorweb exposed through said solidified resin is flush with the surface ofsaid solidified resin.
 3. A method for producing a semiconductor deviceas claimed in claim 1, wherein said metallic heat radiating plate ispositioned in said mold cavity such that the bottom of said metallicheat radiating plate is spaced 0.35 to 0.65 mm from the bottom of saidmold cavity.
 4. A method for producing a semiconductor device as claimedin claim 1, wherein said insulator web is made from an insulatingmaterial having a softening point higher than the temperature at whichsaid resin is charged into said mold cavity.
 5. A method for producing asemiconductor device as claimed in claim 1, wherein said metallic heatradiating plate is provided at said one end thereof with a metallicprojection, said insulator web being attached to said metallicprojection.